cos(π/7) + cos(6π/7)
=cos(π/7) -cos(π-6π/7)
=cos(π/7) -cos(π/7)
=0
利用和差化积公式：
cos(π/7) + cos(6π/7)
=2 cos[(π/7+6π/7)/2] cos[(π/7-6π/7)/2]
=2cos(π/2)cos(5π/14)
=0